Prevention of hydrogen-embrittlement in oxygen-bearing copper



United States Patent 3,352,667 PREVENTION OF HYDROGEN-EMBRITTLEMENT INOXYGEN-BEARING COPPER Dilip K. Das, Bedford, and George Freedman,Wayland, Mass., assignors to Raytheon Company, Lexington, Mass., acorporation of Delaware No Drawing. Filed Sept. 29, 1964, Ser. No.400,246 9 Claims. (Cl. 75-153) This invention relates to the preventionof hydrogen embrittlement in oxygen-bearing metals, and moreparticularly to the production of copper which contains minor amounts ofrefractory metal oxides but which contains substantially no oxygen in aform available for reaction with diffused hydrogen.

It is well known that in some metals there is a tendency toward hydrogenembrittlement when the metal is exposed to a hydrogen-containingatmosphere or other hydrogen-containing environment at elevatedtemperatures. Hydrogen embrittlement may be described as the formationof water vapor within an oxygen-containing metal as a result of theinward diffusion of hydrogen gas into the metal structure, the oxygenbeing in solid solution in the metal or in the form of precipitatednodules of metal oxides (e.g. cuprous oxide) which are readily reducibleby the diffused hydrogen. The hydrogen diffusing into the metal combineswith the oxygen therein to form steam, and the expanding steam weakensthe metal structure at the grain boundaries thereof which causes brittlefailure of the metal when subjected to stress. This can be a seriousproblem. For example, tough pitch copper is a high gradeelectrolytically or pyrometallurgically refined copper containing veryfew metallic impurities and usually between about 0.02 and 0.07% byweight of oxygen predominantly in the form of cuprous oxide. Tough pitchcopper has good electrical conductivity and is relatively easy tofabricate into finished products. However, due to the slight butsignificant free oxygen content (that is, oxygen in a form available forreaction with diffused hydrogen) of this material it is subject tohydrogen embrittlement when heated in the presence of ahydrogen-containing environment (for example, when being brazed with theaid of a conventional acetylene torch or in a hydrogen atmospherefurnace), and therefore the use of tough pitch copper is restricted inpractice by the need to avoid hydrogen embrittlement of this material.

Hydrogen embrittlement of metals, and in particular copper, may beprevented by eliminating as completely as possible the oxygen present inthe copper or other metal that is available for reaction with hydrogen.The production of copper that is virtually oxygen-free is an expensiveand difficult procedure requiring the use of elaborate melting furnacesand casting machines specifically designed to permit deoxidation andprevent reoxidation of the copper. Oxygen free copper of high purity hasexcellent electrical conductivity and can be fabricated by conventionaltechniques in hydrogen-containing atmospheres Without danger of hydrogenembrittlement of the resulting fabricated structure. However, OFHCcopper, as this oxygen-free high conductivity product is called, is asoft and malleable material and has little mechanical strength whenheated, and therefore it is not suitable for products requiringreasonable strength and good dimensional stability at elevatedtemperaturessuch for example as required by high power vacuum tubes andother electronic equipment in which high temperatures are generated bythe electrical energy used by the equipment.

It is known that the physical strength of high purity copper (e.g. OFHCcopper) can be dramatically increased by a process known as dispersionhardening, and the increase in strength and hardness of dispersionhardened copper is retained even at the elevated temperatures at "icewhich such coppers as tough pitch, OFHC, precipitation hardened andother similar coppers would become relatively soft and lacking inmechanical strength. In this process a very small but significantquantity of certain metal oxides such as aluminum oxide, chromium oxide,Zirconium oxide, beryllium oxide and other refractory metal oxides isdispersed throughout the otherwise relatively pure solid copper matrix.The presence of these metal oxides hardens and strengthens the copper,and the thus strengthened copper retains its strength and hardness attemperatures at which other copper and copper alloys soften and losetheir strength.

The actual treatment of the copper required to obtain the desireddispersion of hardening metal oxides therein is complicated anddifficult and may be accomplished in a number of ways. In the mostconvenient process substantially pure copper is alloyed with a small butsignificant quantity of a metal which will form a hardening oxide. Thealloy is then cast, extruded or otherwise formed into a suitable ingotor other shape, and the solid copper alloy is exposed to a source ofoxygen at an elevated temperature in a manner which results in thediffusion of oxygen into the interior of the metal ingot where it reactswith the alloying metal to form the desired refractory metal oxidedispersed through copper matrix. The resulting oxygenated metal ingotcontains refractory oxides which harden the metal and also a small butsignificant quantity of free oxygen predominantly in the form of cuprousoxide that is precipitated and elemental oxygen that is dissolved in themetal. As a consequence, although the dispersion hardened copper hasvastly improved strength, stiffness and hardness even at elevatedtemperatures, it is nonetheless subject to hydrogen embrittlement ifexposed to hydrogen at an elevated temperature due to the presence ofthe free oxygen therein, and in this respect dispersion hardened coppersuffers from the same restrictions as to its use as does tough pitch andother oxygencontaining coppers.

We have now discovered that hydrogen embrittlement of tough pitch,dispersion-hardened and other free oxygencontaining coppers and copperalloys may be substantially completely prevented by treating the copperin the solid state with elemental boron in such a manner thateffectively sequesters the free oxygen present in the copper so thatsubsequent exposure to hydrogen at an elevated temperature has nodeleterious effect. More specifically, we have found that if freeoxygen-containing copper (and by that we mean copper or copper alloyswhich contain oxygen in a form that is available for reaction with diffused hydrogen) is exposed to boron vapor at an elevated temperature,the boron will diffuse into the solid copper matrix and react With thefree oxygen present therein to form precipitated particles of boronoxide dispersed throughout the copper matrix. The boronated coppercontains, in addition, elemental boron dissolved in the solid coppermatrix to the extent that boron is soluble in this metal. Accordingly,our new process for preventing hydrogen embrittlement of freeoxygen-containing copper comprises subjecting the solidoxygen-containing copper to the vapors of elemental boron at atemperature of at least about 800 C. and below the melting point of thecopper for a sufficient time to allow the boron to diffuse into thesolid copper lattice and react with the free oxygen therein and furtherto substantially saturate the copper matrix with elemental boron. Theresulting boronated copper contains boron oxide, substantially no freeoxygen and dissolved elemental boron up to limit of solubility of boronin solid copper, or up to about 0.06% by weight of dissolved elementalboron.

Our new process may be employed to sequester the free oxygen content ofany metal such as copper or copper base alloys that is subject tohydrogen-embrittlement,

and as previously noted it is particularly useful in the treatment oftough pitch copper and dispersion hardened cooper to reduce the freeoxygen content of these materials to levels at least as low as that ofoxygen-free or OFHC copper. Elemental boron readily diffuses into thesolid copper matrix of high purity copper or copper alloys at elevatedtemperatures despite the fact that the solubility of boron in solidcopper is not more than about 0.06% boron by weight. Moreover, boronreacts readily with free oxygen present in the solid metal matrix (thatis oxygen present in the metal in a form that will react with elementalhydrogen that diffuses into the solid metal) to form a stable refractorymetal oxide (B that is not subsequently reducible by diffused hydrogen.As a result, when free oxygen-containing copper is exposed to the vaporsof elemental boron at an elevated temperature, boron will diffuserapidly into the heated copper matrix and will react with the freeoxygen present therein, to form precipitated particles of boron oxide,and the diffusion of elemental boron into the copper will continue untilall of the available oxygen has been sequestered by conversion to boronoxide and until the copper becomes substantially saturated withdissolved elemental boron. The precipitated particles of boron oxidedispersed throughout the copper matrix have no deleterious effect on thedesirable properties of copper, and in fact the boron oxide particlesmay serve to strengthen and harden the copper in the same manner thatthe dispersed particles of refractory metal oxides (such as aluminumoxide, zirconium oxide and the like) serve to strengthen and hardendispersion hardened copper.

The free oxygen-containing copper to be boronated may be exposed to thevapors of elemental boron in any suitable manner. For example, thesolid, oxygen-containing copper part may be heated in an atmosphere ofboron gas at a temperature of, say, 950 C. for approximately one hour.Or, the copper part may be packed in boron powder and the thus packedpart placed in an oven or other suitable heating chamber where it ismaintained at a temperature of, say, between about 800 and 950 C. for asufiicient length of time to insure complete reaction of the free oxygenwith boron. Or, the copper part may be painted with a mixture of boronpowder and a suitable binder (such as nitrocellulose) and the thuspainted part heated at a temperature ofbetween 800 C. and 950 C. for asufficient period of time to insure that the free oxygen content of thecopper is substantially completely sequestered by reaction with boron.The copper part being boronated should be maintained at a temperature ofat least about 800 C. and below the temperature at which copper becomesexcessively soft, and preferably it should be maintained at atemperature between about 800 and 950 C., during the boron treatment.The length of time required to insure substantially complete reactionbetween the diffused boron and the free oxygen content of the copperpart will depend upon the boronation temperature and upon the size andcomposition of the copper part. In general, the time of exposure toboron vapors should be suificient to allow the copper part to becomesubstantially completely saturated with elementboron, and this is amatter that may be determined in each case by observation anddetermination of the elemental boron content of the copper beingtreated.

Tough pitch copper ordinarily contains between about 0.02 and 0.07% byweight of oxygen predominantly in the form of cuprous oxide, a form ofoxygen with which diffused hydrogen will react and which would result inhydrogen embrittlement of the copper unless sequestered in accordancewith our invention. When tough pitch cop peris treated with elementalboron in accordance with our invention, the resulting oxygen-freeproduct contains between about 0.02 and 0.08% by weightof boron oxide,up to about 0.06% by weight of elemental boron and substantially no freeoxygen available for reaction with elemental hydrogen. Similarly,dispersion hardened 4 copper typically may contain up to 0.5 or 0.6% byweight of oxygen predominantly in the form of cuprous oxides hardenedcopper it retains its high strength and hardnessv at elevatedtemperatures. I

The following examples are illustrative but not limitative of thepractice of our invention:

Example I A test strip of electrolytic tough pitch copper containingapproximately 0.04% by weight of oxygen predominantly in the form ofcuprous oxide was packed in boron powder and the thus packed strip wasplaced in an oven maintained at a temperature of 800 C. for a period of2 hours. The resulting boronated copper test strip contained 0.05% byweight of boron oxide and 0.06% by weight of elemental boron dissolvedin the copper matrix. The test strip was then placed in ahydrogen-containing atmosphere at a temperature of 850 C. for a periodof 1 hour. The test strip was then subjected to repeated bendingoperations without fracture or failure due to hydrogenembrittlementafter a dozen 180 reverse .beudings.

A second test strip of electrolytic tough pitch copper containing thesame amount of free oxygen as that of the first test strip was placed inthe same hydrogen-containing atmosphere at the same temperature and forthe same length of time as the first test strip. On removal of thesecond test strip from the hydrogen atmosphere, it was subjected to abending operation. The test strip failed as a result of brittle fracturedue to hydrogen embrittlement after a 45 bend in one direction.

Example 11 A test strip of dispersion-hardened copper containing about0.2% by weight of Zirconium oxide and about.

0.04% by weight of oxygen predominantly in the form of cuprous oxide waspacked in powdered boron and the thus packed copper test strip wasplaced in an oven at a temperature of 800 C. for a period of 2 hours. Oncompletion of the boron treatment, the boronated copper test stripcontained 0.05% by weight of boron oxide, about 0.06% by weight ofelemental boron and substantially no free oxygen available for reactionwith elemental hydrogen. The boronated test strip was then placed in ahydrogen-containing atmosphere at a temperature of 900 C. for a periodof 1 hour. The test strip was then subjected to repeated bendingoperations without failure due to hydrogen embrittlement.

A second test strip of dispersion-hardened copper identical with thefirst test strip was placed in the same hydrogen-containing atmosphereat the same temperature and for the same length of time as the firsttest strip. The resulting hydrogen-treated, dispersion hardened teststrip was then subjected to a bending operation. The test strip failed,due to hydrogen embrittlement, after less than one bending operation.

Example III A test strip of dispersion-hardened copper containing 0.8%by weight of aluminum oxide in finely divided form dispersed uniformlythroughout the solid copper matrix and about 0.04% by weight of freeoxygen predominantly in the form of cuprous oxide was packed in boronpowder, and the thus packed test strip was placed in an oven at atemperature of 800 C. for 2 hours. The boronated test strip contained0.05% by weight of boron oxide, about 0.06% by weight of elemental boronand substantially no oxygen in a form available for reaction withhydrogen, The boronated, dispersion-hardened test. strip was then placedin a hydrogen-containing atmosphere at a temperature of 900 C. at aperiod of 1 hour. The test strip was then subjected to repeated bendingoperations without failure due to hydrogen embrittlement.

A second test strip of dispersion-hardened copper of identicalcomposition with the first test strip was placed in the samehydrogen-containing atmosphere at the same temperature and for the samelength of time as the first test strip, and the resultinghydrogen-treated dispersionhardened copper strip was subjected to abending operation. The test strip failed, due to hydrogen-embrittlement,after less than one complete bending operation.

From the foregoing description of our new procedure for the preventionof hydrogen embrittlement of copper and other such metals it will beseen that we have made an important contribution to the art to which ourinvention relates.

We claim: 1. Process for preventing hydrogen embrittlement ofoxygen-containing copper which comprises subjecting solidoxygen-containing copper to elemental boron at a temperature of at leastabout 800 C. and below the melting point of the metal for a sufiicienttime to allow boron to diffuse into the solid metal lattice and reactwith the oxygen therein, and

recovering a boronated metal product containing boron oxide, dissolvedelemental boron and substantially no oxygen.

2. Process for preventing hydrogen embrittlement of oxygen-containingcopper which comprises subjecting the solid oxygen-containing copper toelemental -boron at a temperature of at least about 800 C. and below themelting point of the metal for a sufiicient time to allow boron todiifuse into the solid copper lattice and react with the oxygen thereinand further to substantially saturate the copper matrix with elementalboron, and

recovering a boronated copper product containing boron oxide, up toabout 0.06% by weight of dissolved elemental boron and substantially nooxygen in a form available for reaction with difiused hydro-gen.

3. Process according to claim 2 in which the oxygencontaining copper isboronated at a temperature of between about 800 and 950 C.

4. Process according to claim 2 in which the oxygencontaining copper issubjected to an atmosphere containing boron vapor.

5. Process according to claim 2 in which the oxygencontaining copper ispacked in elemental boron powder.

6. Process according to claim 2 in which the oxygencontaining copper ispainted with a mixture of boron powder and an inert binder.

7. Dispersion-hardened copper containing boron oxide dispersedthroughout the copper matrix, said boron oxide being present in anamount up to about 0.9%, up to about 0.06% elemental boron in solidsolution in the copper, and substantially no oxygen in a form availablefor reaction with hydrogen.

8. Dispersion-hardened copper according to claim 7 containing fineparticles of a refractory metal oxide dispersed throughout the solidcopper matrix.

9. Dispersion-hardened copper according to claim 8 in which therefractory metal oxide is selected from the group consisting of aluminumoxide, beryllium oxide, chromium oxide and zirconium oxide.

References Cited UNITED STATES PATENTS 845,606 2/1907 Anderson 14813.22,001,017 5/1935 Feussner et al 148-13.2 2,183,592 12/1939 Silliman75--153 2,195,433 4/1940 Silliman 75-153 2,479,311 8/1949 Christensen etal. 75153 X 2,493,951 1/1950 Druyvesteyn et al. 148-132 FOREIGN PATENTS654,962 7/1951 Great Britain.

CHARLES N. LOVELL, Primary Examiner.

7. DISPERSION-HARDENED COPPER CONTAINING BORON OXIDE DISPERSEDTHROUGHOUT THE COPPER MATRIX, SAID BORON OXIDE BEING PRESENT IN ANAMOUNT UP TO ABOUT 0.9%, UP TO ABOUT 0.06% ELEMENTAL BORON IN SOLIDSOLUTION IN THE COPPER, AND SUBSTANTIALLY NO OXYGEN IN A FORM AVAILABLEFOR REACTION WITH HYDROGEN.